There is a 'const' Symbol constructor that is not expressible as a Dart
const constructor because it performs validation of its input. This is
implemented in the libraries by using a dummy const constructor definition
that does not validate its input and replacing calls to 'new Symbol' with
calls to 'new Symbol.validated' in the compiler.
Without replacing calls to 'new Symbol', the non-validating dummy
implementation will be used, which is not correct.
Closes#24878.
R=asgerf@google.com
BUG=https://github.com/dart-lang/sdk/issues/24878
Review URL: https://codereview.chromium.org/1639313002 .
Kevin Millikin
fd9603d1d2